Ultrahigh molecular weight polyethylene (UHMW PE) is well known for, among other properties, its chemical resistance, low friction coefficient, high toughness, and resistance against wear. As a result, it has found numerous applications in demanding environments where the above properties are required, such as in the chemical industry, mining, mechanical engineering, the field of biomedical implants and the textile industry. Equally well known is the intractability of this polymer, which is due to the fact that UHMW PE, above its crystalline melting temperature, does not form a fluid phase that is of a viscosity that permits melt-processing techniques used with common thermoplastic polymers. A negative consequence of the fact that above its crystalline melting temperature UHMW PE merely passes into a viscoelastic state, is that the original polymer particles soften but still retain their shape, and long processing times and/or high temperatures are generally required to process UHMW PE into useful objects. Indeed, even at 200° C. and after prolonged heating, the initial powder morphology often is retained, and generally is present in articles made of UHMW PE. The latter, in fact, is thought to contribute to abrasive wear and ultimate failure of, for example, artificial implants made from this polymer.
The above problems have been recognized since long, and various methods have been proposed to improve processing of UHMW PE, reduce the processing times and associated costs, reduce the granular nature of products made thereof, and generally further improve their properties, notably the resistance against wear. See, for instance, U.S. Pat. No. 5,621,070, U.S. Pat. No. 5,721,334, U.S. Pat. No. 4,587,163, and U.S. Pat. No. 5,037,928. In the methods described in these patents, objects of UHMW PE are produced essentially by compression and sintering under different pressure- and temperature time protocols. The sintering processes described in these patents are comparatively lengthy, however. Another method to process UHMW PE into shapes is described by Corbeij-Kurelec, L. (Ph. D. Thesis, Technische Universiteit Eindhoven, the Netherlands, 2001). In this method, a particular as-polymerized or virgin UHMW PE powder is compacted and subsequently melted. As the author notes, however, the process appears to be not generally applicable and typically limited to polymers based on single-site metallocene catalysts (p. 48).
Other techniques to circumvent the Intractability of UHMW PE include, for example, the addition of solvents, lubricants, plasticizers, and processing aids, as well as polyethylene grades of lower molecular weight. See, e.g., U.S. Pat. Nos. 5,658,992 and 5,422,061. The aforementioned additions of lubricants, plasticizers, and processing aids or lower molecular weight polyethylenes, however, tend to cause a deterioration of some or substantially all of the beneficial properties of UHMW PE.
Unfortunately, the above methods generally are cumbersome, time consuming, not economical or ill defined, and/or severely limit the types and characteristics of objects and products that can be manufactured with UHMW PE.
Thus, a need continues to exist for a method that readily processes ultrahigh molecular weight polyethylenes into coherent articles and exploits the outstanding properties of this unique polymer in a wider and more economical spectrum of product forms.
Accordingly, it is one objective of the present invention to provide UHMW PE compositions of highly beneficial sintering characteristics.
Yet another object is to provide a method to produce UHMW PE compositions of highly beneficial sintering characteristics.
Additionally, it is an object of the present invention to provide a method to process UHMW PE compositions into useful shapes and articles.
Still another object of the present invention is to provide useful shapes and articles that are manufactured by sintering of UHMW PE compositions.